The VMH and Molecular Control of Energy Balance The ventral medial hypothalamus (VMH), in concert with other brain regions, regulates energy balance. A group of neurons within the VMH, marked by the expression of the transcription factor, SF1 (Nr5a1), resists the development of obesity. The adipocyte-derived, catabolic hormone leptin excites these SF1 neurons, and deletion of leptin receptors (LEPRs) on SF1 neurons results in obesity, and also marked sensitivity to diet-induced obesity. Thus, SF1 neurons in the VMH, like POMC neurons in the actuate nucleus, are direct targets of leptin and promote negative energy balance. While much is known about how POMC neurons cause weight loss, comparatively less is known about how SF1 neurons achieve this effect. This presents the unique opportunity to identify novel mechanisms controlling energy homeostasis. In this grant, we propose to determine the following: 1) the catabolic factor released by SF1 neurons (we propose an important role for the neuropeptide, PACAP) (Aim 1), 2) the receptor and neuron immediately downstream of SF1 neurons (we propose that the PAC1-receptor - the high affinity PACAP receptor - on POMC neurons is important) (Aim 2), and 3) on the afferent side of SF1 neurons, we propose that glutamatergic excitatory inputs, which synapse on dendritic spines of SF1 neurons, play a key role in controlling the activity of SF1 neurons, and that glutamate NMDA receptor-mediated plasticity of these inputs, and, importantly, leptin regulation of this plasticity, contributes prominently to the control of energy homeostasis. To accomplish these Aims, we will utilize the following state-of-the-art technologies: 1) neuron- specific gene manipulation to test molecular mechanisms in an in vivo context (in all three Aims), 2) optogenetics to identify functionally relevant, monosynaptic, downstream targets of SF1 neurons (in Aim 2), and 3) functional and morphologic assessments of excitatory synaptic plasticity (in Aim 3).